CCD image sensors are widely used to convert images into electronic signals that can be captured, transmitted, stored and displayed. Digital imaging devices such as digital image scanners, digital video cameras and digital still cameras typically use CCDs.
A CCD divides an image into a large number of discrete cells or pixels that are often arranged as an array of rows or lines. During an exposure interval, each pixel in the array is arranged to convert incident light into electrons, which form a current that is integrated by a capacitor. The current flow is proportional to the intensity of the exposure of the pixel. At the end of the exposure interval, the accumulated charge for each pixel in the array corresponds to the scanned image. Prior to the next exposure interval, the pixels are reset to the “reset level” by initializing the capacitors to a specified voltage. The reset level and the charge accumulated for each pixel are transferred to an output port so that the resulting image can be processed.
An analog processing stage senses the output signal and removes the reset level from the image during a readout interval. The reset level varies over temperature due to thermal noise, and also varies from part to part due to processing variations. The reset level represents the voltage that is present when no light is incident on a pixel (i.e., the black level for the pixel). During the readout interval, the output signal includes the reset level followed by the signal level for the pixel. The difference between the signal level and the reset level indicates the amount of light (typically of a particular color) that has been impressed upon one particular pixel of the image sensor.